Organic compounds absorb light, and thereby the compounds are converted to be in an excited state. By going through this excited state, such organic compounds generate various reactions (such as photochemical reactions) in some cases, or luminescence is produced in some cases. Therefore, various applications of the organic compounds have been being made.
As one example of the photochemical reactions, a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule is known (refer to Reference 1: Haruo INOUE, et al., Basic Chemistry Course PHOTOCHEMISTRY I (Maruzen Co., Ltd.), pp. 106-110, for example). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by direct photoexcitation. However, singlet oxygen is generated in the presence of any other triplet excited molecule, which leads to an oxygen addition reaction. In this case, a compound capable of forming the triplet excited molecule is referred to as a photosensitizer.
As described above, in order to generate singlet oxygen, a photosensitizer that is capable of forming a triplet excited molecule by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition, and a triplet excited molecule is hardly generated. Therefore, as such a photosensitizer, a compound which easily generates intersystem crossing from the singlet excited state to the triplet excited state (or a compound which allows the forbidden transition of photoexcitation directly to the triplet excited state) is required. In other words, such a compound can be used as a photosensitizer and is useful.
Also, such a compound often emits phosphorescence. The phosphorescence is luminescence generated by transition between different energies in multiplicity and in the case of an ordinary organic compound, indicates luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, luminescence in returning from a singlet excited state to a singlet ground state is referred to as fluorescence). Application fields of a compound capable of emitting phosphorescence, that is, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light-emitting element using an organic compound as a light-emitting substance.
This light-emitting element has a simple structure in which a light-emitting layer containing an organic compound that is a light-emitting substance is provided between electrodes, and has attracted attention as a next-generation flat panel display element because of its characteristics such as a thin shape, lightweight, high response speed, and low direct current voltage driving. In addition, a display device using this light-emitting element is superior in contrast, image quality, and wide viewing angle.
The emission mechanism of a light-emitting element in which an organic compound is used as a light-emitting substance is a carrier injection type. That is, by applying voltage with a light-emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make the light-emitting substance excited, and light is emitted when the excited state returns to the ground state. As in the case of photoexcitation described above, types of the excited state include a singlet excited state (S*) and a triplet excited state (T*). Further, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
At room temperature, as for a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound), only luminescence from the singlet excited state (fluorescence) is observed, but luminescence from the triplet excited state (phosphorescence) is not observed. Therefore, in a light-emitting element using a fluorescent compound, the theoretical limit of internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% based on S*:T*=1:3.
On the other hand, when the phosphorescent compound described above is used, the internal quantum efficiency can be improved to 75 to 100% in theory. Namely, a light emission efficiency that is 3 to 4 times as much as that of the fluorescence compound can be achieved. For these reasons, in order to achieve a highly-efficient light-emitting element, a light-emitting element using a phosphorescent compound has been developed actively (for example, refer to Reference 2: Zhang, Guo-Lin, et al., Gaodeng Xuexiao Huaxue Xuebao (2004), vol. 25, No. 3, pp. 397-400). In particular, as the phosphorescent compound, an organometallic complex using iridium or the like as a central metal has been attracting attention, owing to its high phosphorescence quantum yield.